The present invention relates to a system for controlling air-fuel ratio of an air-fuel mixture for an automotive engine, and more particularly to a system for controlling the air-fuel ratio in accordance with a feedback signal from an o.sub.2 -sensor for detecting the oxygen concentration of the exhaust gases.
Generally, the engine is provided with a carbon canister for absorbing the fuel vapor in a fuel tank during the time that the engine is not running and for purging the fuel vapor in the canister to an intake manifold under predetermined conditions of the engine operation. When the fuel in the canister is purged, the fuel vapor is added to the air-fuel mixture induced into the cylinders of the engine, rendering the mixture rich.
The air-fuel ratio control system operates to dilute the rich mixture in accordance with the feedback signal of the O.sub.2 -sensor. However, since the deviation of the air-fuel mixture is large compared with the deviation which may occur in the steady state condition of the engine, it takes a long time to control the deviated air-fuel ratio back to the stoichiometric air-fuel ratio.
Heretofore, there is no control of deviation of the air-fuel ratio during purging of the fuel vapor.